The present invention relates to a hermetic scroll compressor which is suitable for use as a refrigerant compressor in an air conditioning system or as a compressor for compressing helium gas. More particularly, the invention is concerned with a hermetic scroll compressor having a hermetic casing which has oil separating function.
The specification of U.S. Pat. No. 4,545,747 discloses a hermetic scroll compressor having a motor-driven scroll compressor section encased by a hermetic casing, particularly an improved construction of the hermetic casing. In this hermetic scroll compressor, a motor-compressor unit constituted by an electric motor and a scroll compressor which are assembled together in a unit is encased by the hermetic casing which is composed of three sections, namely, an upper casing section, a barrel section and a lower casing section. The motor-compressor unit is mounted on a frame which is secured in the casing seemingly in close contact with the inner surface of the casing wall. More specifically, as shown in FIG. 8, the outer periphery 11m of the frame 11 is shrink-fitted in the inner periphery of the barrel section 1b of the casing. The barrel section 1b of the casing is formed by rolling a thin sheet material into a cylindrical form and welding the opposing ends of the thus rolled material by electric arc welding so as to form a weld joint.
As will be seen form FIG. 9, the weld seam joint 1m has a portion formed to project inwardly from the casing as at 1m'. This projection 1m' provides an essential thickness for attaining the required strength of the weld joint. During the welding, portions of the casing on both side of the weld seam joint 1m are deformed under the influence of the projection 1m' so that small clearances are formed as at 11p, 11q between these portions of the casing and the outer peripheral surface 11m of the frame 11. These clearances extend in the axial direction of the casing over the length of the barrel section 1b of the casing. Thus, the weld seam joint 1m forms a step on the inner surface of the barrel section 1b as shown in FIG. 9. Accordingly, a discharge chamber and a motor chamber, which are partitioned by the frame, are communicated with each other not only through a designed communication passage 18 but also through these clearances, thus impairing the seal between these two chambers.
In the case where a discharge pipe 19 is connected by welding to the portion of the barrel section 1b near the weld seam joint 1m, the sizes of the clearances 11p, 11q are increased due to heat applied to the barrel section during welding.
The communication passage 18 constituted by passage portions 18a and 18b (see FIG. 10) is provided intentionally for the purpose of allowing refrigerant gas and oil to pass therethrough. However, the oil leaks from the upper discharge chamber 2a into the lower chamber 2b through the clearances 11p, 11q around the weld seam joint 1m. oil leaking through the clearances 11p, 11q directly flows in the discharge side. In particular, when the weld seam joint 1m is disposed in the vicinity of the discharge pipe 19 as shown in FIG. 9, the oil leaking through the clearances 11p, 11q can easily reach the discharge pipe 19. The oil is then discharged directly to the outside of the compressor, as shown by broken-line arrows in FIG. 10. The designed circulation of the oil in the compressor is not described herein because it does not constitute any critical portion of the invention.
In the operation of the compressor described above, the oil is introduced together with the refrigerant gas from the upper discharge chamber 2a into the lower chamber 2b through the communication passage portions 18a, 18b. Since the lower chamber 2b has an ample volume, the gas suspending the oil is expanded and decelerated therein so that the oil suspended by the gas is separated from the latter. The oil separation efficiency of the casing, however, is impaired considerably, when the aforementioned clearances 11p, 11q are disposed in the vicinity of the discharge pipe 19, or when the same are located circumferentially intermediate between the communication passage portions 18a, 18b and the discharge pipe 19. The reason for this is that in such a case a significant portion of the oil is conveyed to the outside of the compressor without being separated from the gas. The rate of leak of the oil through the clearances 11p, 11q is increased in proportion to an increase in the pressure differential between the upper discharge chamber 2a and the lower chamber 2b, so that the rate of convey of the oil to the outside of the casing is increased as the scroll compressor is driven at high speed by an inverter. In addition, the sizes of the clearances 11p, 11q are not constant but vary depending on the amount of thermal deformation of the casing during the welding. This means that different compressors exhibit different rates of convey of oil to the outside of the compressors through the clearances.
Thus, the clearances formed on both sides of the weld seam joint impaires the oil separating efficiency of a hermetic scroll compressor and causes fluctuation of rate of convey of the oil to the outside of the compressor. The elimination of leak of oil through these clearances, therefore, is a serious problem also from the view point of the product quality control.